linux-kernel - Re: [PATCH v4] io_uring: reduce latency by reissueing the operation

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Message-ID: <1a6a8eba-96e3-0afb-0357-3ac3b08cba36@gmail.com>
Date:   Tue, 22 Jun 2021 21:51:28 +0100
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Olivier Langlois <olivier@...llion01.com>,
        Jens Axboe <axboe@...nel.dk>, io-uring@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] io_uring: reduce latency by reissueing the operation

On 6/22/21 8:05 PM, Olivier Langlois wrote:
> On Tue, 2021-06-22 at 19:01 +0100, Pavel Begunkov wrote:
>> On 6/22/21 6:54 PM, Pavel Begunkov wrote:
>>> On 6/22/21 1:17 PM, Olivier Langlois wrote:
>>>>
>>>
>>>>  static bool __io_poll_remove_one(struct io_kiocb *req,
>>>> @@ -6437,6 +6445,7 @@ static void __io_queue_sqe(struct io_kiocb
>>>> *req)
>>>>         struct io_kiocb *linked_timeout =
>>>> io_prep_linked_timeout(req);
>>>>         int ret;
>>>>  
>>>> +issue_sqe:
>>>>         ret = io_issue_sqe(req,
>>>> IO_URING_F_NONBLOCK|IO_URING_F_COMPLETE_DEFER);
>>>>  
>>>>         /*
>>>> @@ -6456,12 +6465,16 @@ static void __io_queue_sqe(struct
>>>> io_kiocb *req)
>>>>                         io_put_req(req);
>>>>                 }
>>>>         } else if (ret == -EAGAIN && !(req->flags &
>>>> REQ_F_NOWAIT)) {
>>>> -               if (!io_arm_poll_handler(req)) {
>>>> +               switch (io_arm_poll_handler(req)) {
>>>> +               case IO_APOLL_READY:
>>>> +                       goto issue_sqe;
>>>> +               case IO_APOLL_ABORTED:
>>>>                         /*
>>>>                          * Queued up for async execution, worker
>>>> will release
>>>>                          * submit reference when the iocb is
>>>> actually submitted.
>>>>                          */
>>>>                         io_queue_async_work(req);
>>>> +                       break;
>>>
>>> Hmm, why there is a new break here? It will miscount
>>> @linked_timeout
>>> if you do that. Every io_prep_linked_timeout() should be matched
>>> with
>>> io_queue_linked_timeout().
>>
>> Never mind, I said some nonsense and apparently need some coffee
> 
> but this is a pertinant question, imho. I guess that you could get away

It appeared to me that it doesn't go down to the end of the function
but returns or so, that's the nonsense part.

> without it since it is the last case of the switch statement... I am
> not sure what kernel coding standard says about that.

breaks are preferable, and falling through should be explicitly
marked with fallthrough;
 
> However, I can tell you that there was also a break statement at the
> end of the case for IO_APOLL_READY and checkpatch.pl did complain about
> it saying that it was useless since it was following a goto statement.
> Therefore, I did remove that one.
> 
> checkpatch.pl did remain silent about the other remaining break. Hence
> this is why I left it there.

-- 
Pavel Begunkov